1. Field of the Invention
The present invention relates to signal tracking. More particularly, the invention relates to a method and apparatus for reducing geolocation ambiguity in signal tracking that compares the results of at least two geolocation algorithms to reduce geolocation ambiguities.
2. Description of the Related Art
The ability to determine the source geolocation of emitted signals is becoming increasingly important as the use of wireless communications devices becomes commonplace throughout the world. For example, the U.S. Federal Communications Commission Enhanced 911 (E911) rules will eventually require cellular telephone carriers to identify the geolocations, i.e. the physical source locations, of subscribers who place calls to 911 or other emergency services. Additionally, wireless communication device users often desire to acquire accurate geolocations for navigation purposes, such as to generate a route between a current location and a destination. Further, military and law enforcement agencies often desire to locate sources of emitted signals for tracking and targeting purposes.
Methods and devices have been developed that enable signal geolocations to be determined. Some of these methods include utilizing Global Position System (GPS) elements that must be coupled with signal emitters to determine geolocations, thereby increasing system cost and complexity. Other methods include utilizing one or more collector elements, such as antennas, to generate signal measurements and compute geolocations utilizing the generated signal measurements.
Although utilizing signal measurements enables geolocations to be determined without interfering with signal emitters, the algorithms that utilize signal measurements to determine geolocations produce ambiguous results that often lead to incorrect and inaccurate geolocation determinations. For instance, the often utilized TDOA/TDOA and TDOA/FDOA algorithms each generate a plurality of geolocations, only one of which is an accurate geolocation.
To reduce the number of ambiguities, users generally are required to utilize prior signal information or direction-finding (DF) methods to exclude one or more of the ambiguous results. For instance, after a set of potential geolocations is provided by a TDOA/TDOA algorithm, users may determine the direction of the signal, such as with angle-of-arrival (AOA) or direction-of-arrival (DOA) measurements, and eliminate potential geolocations not corresponding to the determined angle or direction.
Although such methods are generally effective, they are time-consuming and costly as they require knowledge of previous signal characteristics, additional calculations, and/or special hardware to ascertain the direction of the emitted signal. Thus, users are often unable to reduce ambiguous geolocations provided by geolocation algorithms due to the cost and complexity of DF methods and the unavailability of prior signal information.